The present invention relates to a gamma camera for detecting a gamma ray radiated from radio-isotope (hereinafter referred to as RIs) administered to a human subject and imaging an internal concentration distribution of the RIs.
The gamma camera is classified into a type of imaging using a single photon nuclide emitting one photon at a decay of the RIs and a type of imaging using a positron nuclide emitting a pair of photons in opposite directions at a quenching of a positron. Recently, the imaging method has been diversified to cover the following methods.
(Static Imaging)
The imaging method is to obtain RIs distribution (plane image) by detecting photons, in a predetermined time period, with a single camera head fixed to a human subject and counting them.
(SPECT Imaging)
This imaging method comprises rotating one camera head around the human subject, while repeating the detection and counting of photons, and reconstructing RIs distribution (cross-sectional image), as in a CT scanning, on the basis of a count value obtained.
(Two Camera Heads-Opposed SPECT Imaging)
This imaging method comprises rotating two camera heads oppositely arranged with a human subject located therebetween while maintaining this positional relation, detecting/counting photons during this time period, and reconstructing a cross-sectional image, as in a CT scanning, on the basis of a count value obtained.
(Two Camera Head 90.degree. Displaced SPECT Imaging)
This imaging method comprises rotating two camera head 90.degree. displaced around a rotation axis while maintaining this positional relation, repeating the detection and counting of photons during this time period, and reconstructing a cross-sectional image, as in a CT scanning, on the basis of a count value obtained.
(Three Camera Head SPECT Imaging)
This imaging method comprises arranging three camera heads in a triangular array, rotating these cameras around a human subject while rotating these cameras while maintaining this positional relation, repeating the detection and counting of photons during this time period, and reconstructing a cross-sectional image, as in a CT scanning, on the basis of a count value obtained.
In these various imaging methods, in order to improve the image quality and quantitative property, various corrections are required, such as an energy correction, a linear correction for correcting a deformation in a marginal edge of a visual field, a uniformity correction for uniforming a variation in sensitivity of a photomultiplier, a scattering ray correction for eliminating scattering components, a crosstalk correction for correcting a crosstalk between two kinds of RIs differing in their photoelectric peaks, an absorption correction for correcting a count error resulting from the non-uniform coefficient of a living body, and so on.
A triple energy window (TEW) method is an excellent correction technique effective to not only the scattering correction but also a crosstalk correction. The TEW method requires three energy windows. The three energy window comprises, as shown in FIG. 1, one main window and two sub-windows. The main window has its center arranged at a photoelectric peak (Epeak) of a target nuclide. The two sub-windows are arranged one at each near side of the main window.
A scattering component (cross-hatched section) mixing into the main window is estimated by a trapezium approximation calculation from a calculated value of the two sub-windows. The estimated scattering component is subtracted from the calculated value of the main window. From the RIs it is possible to obtain the number of primary photons involved.
The greater the width of the main window, an image can be formed with many more photons. If the width of the main window is too greater, an amount of scattering lines mixed becomes greater, thus resulting in a lowering in an S/N ratio. It has been conventional practice to set the width of the main window to be 20% of the photoelectric peak Epeak corresponding to a maximum frequency or that of the sub-window to be 7% of the photoelectric peak Epeak, not depending upon the nuclide involved.
In the conventional method by which the window width is uniformly set in this way, there is a tendency that the width becomes too narrow at a relatively low photoelectric peak, for example, TI-201. This has been thus far indicated, but the setting method for optimizing these windows for respective nuclides has not yet currently established.
In order to make the above-mentioned absorption correction, it is necessary that the spatial distribution of an absorption coefficient on the human subject be measured with the use of an external ray source of a spatially uniform photon radiation frequency. The spatial distribution of the absorption coefficient can be found as follows. That is, the photons radiated from the external ray source are detected with a camera head after they have been transmitted through the human subject. The counting of only the photons whose energies are in the windows is made for respective incident positions and this is continued for a predetermined time period. The number of photons emitted for this time period from the external ray source is known and, being given as "I.sub.0 ", then a relation below is established: EQU I.sub.1 =I.sub.0.multidot.e.sup.-.mu..multidot.d
where
I.sub.1 : the number of photons transmitted through the human being, that is, a count value; PA1 .mu.: the absorption efficient; and PA1 d: the thickness of the human subject.
From this relation it is possible to find the absorption coefficient .mu..
By correcting a count value of the photons from the RIs, administered into the human subject, on the basis of the absorption coefficient it is possible to compensate for a count error resulting from a difference of the absorption coefficient.
In order to correct the scattering ray, the TEW method has been used even in finding a spatial distribution of the absorption coefficient. Since, however, the window is not optimized as set out above, there is a strong tendency that, in this case, the scattering ray is principally underestimated, thus presenting the problem of the absorption coefficient being lower than in an actual instance.